Research-grade CTD Profile to 2,000m in 50 Minutes

Made possible by some hardware upgrades, the record-breaking system on the Bluefin allows the NDBC group to gather high-quality CTD profiles, even though the vessel is not equipped with a conventional deepwater CTD winch.

NDBC technicians visit the DART tsunami warning moorings in the North Pacific Ocean annually to conduct routine maintenance. DART moorings are taut, with a scope of 0.97 and are very sensitive to snap loading so water depth must be known to highest degree of accuracy possible. An echo sounder is used to determine the exact water depth at each mooring location, but an accurate CTD-derived sound speed profile is also required. As the maintenance cruises are not conducted on oceanographic research vessels equipped with deepwater CTD winches, the NDBC team needed a portable system that could be easily installed to gather the high-quality and deepwater CTD profiles they needed and the UnderwayCTD was selected as the best fit. A key requirement was the capability to cover most of the water depth with the CTD profile: 2,000m was specified as the maximum required profile depth. The NDBC’s UnderwayCTD was used first in 2011, and while deep profiles were achieved there was a requirement for a faster drop speed to approach the target 2,000m mark. Oceanscience engineers were able to implement an improved line loading regime for the UnderwayCTD’s unique probe tail spool, leading to a sustained CTD fall rate of over 3.5m/s for the first 1,100m of the descent. The NDBC technicians were among the first groups to test the new system, and the improvement was obvious. The 2012 mooring cruise saw the UnderwayCTD depth record broken four times in a row Then, at DART Station 52405, positioned at 12° 52' 53" N, 132° 20' 02" E with no currents to drift the probe, NDBC technicians were able to send the CTD down to 2,041m. According to James Coleman of NOAA said the team was determined to capture the depth record. However, even they were surprised discovering that they had exceeded 2,000m.

Although it is not possible to go much deeper, upcoming further development of the UnderwayCTD promises to make the profiling process even faster.